Since the topic came up recently in the post about the Blood God, and I didn't want to derail that topic, I was wondering a bit about Huec sacrificial practices.

First of all, are there only certain gods they offer sacrifices to, or is all of the pantheon eligible? I know for certain that Qetzicotl is offered human sacrifices, but I'm not sure about any others.

Are humans the only types of sacrifices made, or do they perform sacrificial ceremonies with animals or food or herbs sacroficed/burned as well for more common ceremonies, or for less powerful intervention? If so, are these things that the average Huec would do themselves or in groups, or are they things only the priesthood would do?

What happens if an Undying is sacrificed? Is it seen as an omen? Are there any times Undying individuals would purposely be sacrificed, or are they generally avoided as offerings? Would they ever be sacrificed in an attempt to prevent their soul from returning?

are there only certain gods [Huecs] offer sacrifices to, or is all of the pantheon eligible?

Any god considered by the Huecs to be a splinter of Toteoh are eligible, as well as Toteoh himself in a broader sense. That is to say, pretty much any deity, as the Huecs pretty much assume any deity that exists is a splinter of Toteoh. Exceptions would be Ascended, like Crowhaven, Owleyes, and Kebyet - they're specifically believed to have been mortal once, and they weren't even Huecs, so they're not related to Toteoh.

Are humans the only types of sacrifices made, or do they perform sacrificial ceremonies with animals or food or herbs sacroficed/burned as well for more common ceremonies, or for less powerful intervention?

Offerings of food or herbs or treasure or anything, really, could be made, but they would just be offerings, and not really expected to have any resulting effects, since they have no spirits to go on and empower/be empowered by the various Huec deities. Animals might be used as supplementary sacrifices by some, but it wouldn't be common, and without a human involved they'd likely be considered worthless as sacrifices, and seen as offerings instead.

are these things that the average Huec would do themselves or in groups, or are they things only the priesthood would do?

Offerings could be made by anyone. Sacrifices are only to be performed by the priesthood.

What happens if an Undying is sacrificed?

If someone came back after a sacrifice (Undying), that would be seen as a rejection - so rejected in fact that the soul was kicked back into the mortal world. Ouch. Whether that was because the soul was bad/unworthy in some way, or because the purpose of the sacrifice was rejected by the god/gods, would be up to the priesthood to determine.